Extensive researches are performed in the field of electronic filters since the significant development of mobile communication which calls for very effective RF filtering circuits.
Therefore, in accordance with the more recent mobile communications of the type UMTS, 3G or WCDMA, it is necessary to extract different closely related channels within a same range of frequencies.
To achieve this, it is necessary to use very selective filters with small levels of insertion losses. Acoustic resonators, which offer a high level of quality factor, show to be a major component for the design of such filtering circuits.
Two particular structures have been classically used in the art, the so-called “ladder” structure and the “lattice” structure.
FIG. 1 recalls the general architecture of one “lattice” structure which comprises first and second series impedances Zs, 100 and 200, with third and fourth parallel impedances Zp 300 and 400.
Generally speaking, in order to compensate for the internal dispersions of the components which constitute the filter, the series impedance Zs and the parallel impedance Zp are carried out by a tunable resonating circuit, generally under control of an electric quantity.
The tuning of the different components of the “lattice” structure is then achieved by a sophisticated adaptive control loop which allows the filter to fit the desirable requirements, despite the inaccuracies resulting from the manufacturing processes, the aging of the electronic components, and the variation of temperature as well as that of the power voltage.
Several adaptive mechanisms are known in the art.
The following documents disclose techniques which are already known for tuning a lattice filter, based on one master-slave architecture.
“Design Considerations for High-Frequency Continuous-Time Filters and Implementation of an Antialiasing Filter for Digital Video”, Venugopal Gopinathan, Yannis P. Tsividis, Khen-Sang Tan, Richard K. Hester, IEEE, 1990.
“Design Techniques for automatically Tuned Integrated Gigahertz-Range Active LC Filters”, Dandan Li, Yannis Fellow, IEEE, 2002.
The following document discloses another technique for tuning a lattice filter:
“Automatic tuning of frequency and Q-factor of bandpass filters based on envelope detection”, Aydin Ilker Karplayan and Rolf Schaumann, IEEE, 1998.
French patent application no. FR 04 03492 filed on Apr. 2, 2004 by the assignee of this application (and which corresponds to U.S. Pat. No. 7,187,240) also discloses the principle of a master-slave adaptive control of a filtering circuit based on a BAW resonator allowing the tuning of the resonance parameters of a “slave” filter with respect to the parameters of a dual “master” filter, the latter being incorporated within the adaptive control loop.
French patent application no. FR 04 03493 filed on Apr. 2, 2004 by the assignee of the present application (and which corresponds to U.S. Pat. No. 7,345,554) describes the incorporation of the “master” structure within one adaptive control loop of the type Phase Locked Loop (P.L.L.).
French patent application no. FR 04 03494 also filed on Apr. 2, 2004 by the assignee of the present application (and which corresponds to U.S. Pat. No. 7,218,181) discloses the integration of the “master” structure within an Amplitude Locked Loop (ALL).
At high frequencies, one particular critical issue has appeared, e.g., the fact that the circuits composing the direct environment of the resonators introduce many parasitic elements (parasitic capacitances etc.) spoiling the ideal working operation, and the principle according to which the “master” structure should be the ideal image of the “slave” structure, thus ascertaining a perfect adaptive control process.
The filtering effect of the lattice filter is achieved by the resonance of elements Zs and Zp, and their cooperation. Elements Zs and Zp sometimes show multiples resonance frequencies, some of them being useful for the filtering process, and others being not.